Flex (columnaris) and livebearers.

[Allaquatica]

Probably the biggest enemy of the guppy besides the uncycled tank, Columnaris or 'flex' probably accounts for between 80-90% of disease related guppy deaths in the home aquarium. It usually starts as a tiny white ares near one of the fins (any) or the moth and eyes and rapidly spreads as a pale slightly pale or 'coocked' looking patch over the fishes body. the fish can become paralized and die within hours of showing symptoms. It can also appear as a range of less severe fin leasions and white fuzzies that are relativley easy to treat. I find the greatest cause of these problems are uncycled tanks, water too hot (over 26.c) and contamination from overfeeding.

a good article: www.flippersandfins.net/flexibacter.htm

[csigirl]

Great article! Dont forget overcrowding. I learned about FC the hard way

[Allaquatica]

low oxygen can also be a problem. Waste from too many fish and the low oxygen in an overcrowded tank would definitley cause it.